Alkenes. Introduction—Structure and Bonding. Alkenes are also called olefins . Alkenes contain a carbon—carbon double bond. Terminal alkenes have the double bond at the end of the carbon chain. Internal alkenes have at least one carbon atom bonded to each end of the double bond.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Introduction—Structure and Bonding
Bond dissociation energies of the C—C bonds in ethane (a bond only) and ethylene (one and one bond) can be used to estimate the strength of the component of the double bond.
Cycloalkenes having fewer than eight carbon atoms have a cis geometry. A trans cycloalkene must have a carbon chain long enough to connect the ends of the double bond without introducing too much strain.
Compounds with two double bonds are named as dienes by changing the “-ane” ending of the parent alkane to the suffix “–adiene”. Compounds with three double bonds are named as trienes, and so forth.
Naming an alkene in which
the longest carbon chain does
not contain both atoms of the
Examples of cycloalkene
Naming alkenes with common
A consequence of this dipole is that cis and trans isomeric alkenes often have somewhat different physical properties.
Also recall that these elimination reactions are stereoselective and regioselective, so the most stable alkene is usually formed as the major product.
Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides.
Five addition reactions of
The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. In step 1, the alkene is the Lewis base that donates an electron pair to H—Br, the Lewis acid, while in step 2, Br¯ is the Lewis base that donates an electron pair to the carbocation, the Lewis acid.
The basis of Markovnikov’s rule is the formation of a carbocation in the rate-determining step of the mechanism.
According to the Hammond postulate, Path  is faster because formation of the carbocation is an endothermic process. Thus, the transition state to form the more stable 2°carbocation is lower in energy.
Electrophilic addition and the
A racemic mixture
The mechanism of hydrohalogenation illustrates why two enantiomers are formed. Initial addition of H+occurs from either side of the planar double bond.
Nucleophilic attack of Cl¯ on the trigonal planar carbocation also occurs from two different directions, forming two products, A and B, having a new stereogenic center.
1,2-dimethylcyclohexene with HCI
Alcohols add to alkenes, forming ethers by the same mechanism. For example, addition of CH3OH to 2-methylpropene, forms tert-butyl methyl ether (MTBE), a high octane fuel additive.
Carbocations are unstable because they have only six electrons around carbon. Halonium ions are unstable because of ring strain.
In the second step, nucleophilic attack of Cl¯ must occur from the backside.
cis-2-Butene yields two enantiomers, whereas trans-2-butene yields a single achiralmeso compound.
Halogenation of cis- and
Treatment of an alkene with a halogen X2 and H2O forms a halohydrin by addition of the elements of X and OH to the double bond.
Even though X¯ is formed in step  of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X¯ is the nucleophile.
Although the combination of Br2 and H2O effectively forms bromohydrins from alkenes, other reagents can also be used.
Because the bridged halonium ion is opened by backside attack of H2O, addition of X and OH occurs in an anti fashion and trans products are formed.
With unsymmetrical alkenes, the preferred product has the electrophile X+ bonded to the less substituted carbon, and the nucleophile (H2O) bonded to the more substituted carbon.
As in the acid catalyzed ring opening of epoxides, nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate the partial positive charge in the transition state.
Hydroboration—oxidation is a two-step reaction sequence that converts an alkene into an alcohol.
BH3 is a reactive gas that exists mostly as a dimer, diborane (B2H6). Borane is a strong Lewis acid that reacts readily with Lewis bases. For ease of handling in the laboratory, it is commonly used as a complex with tetrahydrofuran (THF).
The first step in hydroboration—oxidation is the addition of the elements of H and BH2 to the bond of the alkene, forming an intermediate alkylborane.
The proposed mechanism involves concerted addition of H and BH2 from the same side of the planar double bond: the bond and H—BH2 bond are broken as two new bonds are formed.
Conversion of BH3
to a trialkylborane
Because the alkylborane formed by the reaction with one equivalent of alkene still has two B—H bonds, it can react with two more equivalents of alkene to form a trialkylborane.
Since only one B—H bond is needed for hydroboration, commercially available dialkylboranes having the general structure R2BH are sometimes used instead of BH3. A common example is 9-borabicyclo[3.3.1]nonane (9-BBN).
With unsymmetrical alkenes, the boron atom bonds to the less substituted carbon atom.
This regioselectivity can be explained by considering steric factors. The larger boron atom bonds to the less sterically hindered, more accessible carbon atom.
Hydroboration of an
Since alkylboranes react rapidly with water and spontaneously burn when exposed to air, they are oxidized, without isolation, with basic hydrogen peroxide (H2O2, ¯OH).
Suppose we wish to synthesize 1,2-dibromocyclohexane from cyclohexanol.
To solve this problem we must:
Working backwards from the product to determine the starting material from which it is made is called retrosynthetic analysis.